The reference design for the Wiznet W6100 Ethernet Shield (https://github.com/Wiznet/Hardware-Files-of-WIZnet/tree/master/08_OSHW/W6100%20Ethernet%20Shield) includes a jumper on pin MOD, with a label in the schematic that says:
Mode Setting Pin Header
Short: W6100 Mode - Default
Open: W5500 mode
What does this mean?
The W5500 and W6100 aren’t pin-compatible, so they can’t go on the same PCB. In my testing, using MOD to put the W6100 into W5500 mode as shown in the datasheet simply makes the chip stop responding. Based on the label in the schematic, it sounds like this should make the W6100 operate in the same way as a W5500, but it doesn’t.